This invention relates to a process for die casting metal that includes driving a plunger into metal within a die cavity during solidification to densify the metal. More particularly, this invention relates to utilizing plunger densification in die casting a metal article adapted to have a bore completely therethrough surrounded by dense, pore-free metal.
In die casting aluminum or similar metals, molten metal is injected into a die cavity and cooled to solidify it to form a product casting. Although molten metal initially fills the cavity, the volume of metal shrinks as it cools and solidifies, creating empty pores in the casting. Shrink pores are particularly formed in more massive, slower cooling sections where the metal solidifies last. One method for reducing shrink porosity employs a plunger to squeeze the metal within the die cavity to collapse the pores. The plunger is initially retracted in a channel that opens into the cavity and squeezes metal out from the channel using a substantially flat tip. This is principally effective for densifying the metal in the direction of the plunger, but has not been satisfactory for assuring densification in surrounding or more remote metal.
In casting articles such as transmission pump covers, the casting comprises an enlarged section that is subsequently drilled to form a bore extending completely through the casting. The use of cores during casting to form a portion of the bore is desired to reduce the machining required to finish the bore. It is also desired that the finished bore be surrounded by dense, nonporous metal along its entire length, which is complicated because shrink porosity is more severe in the enlarged section.
Therefore, it is an object of this invention to provide an improved method and apparatus for casting metal to form an article adapted to have a bore completely therethrough surrounded by densified metal substantially free of shrink porosity, which casting is carried out in a metal molding cavity comprising core members that form recesses in the casting that provide the basis for the completed bore and densify the metal intended to lie about the bore along its entire length.
More particularly, it is an object of this invention to provide coring within a metal molding cavity of a die casting apparatus and comprising opposed core members that are adapted to be driven together during casting while the metal is partially solidified and in an extrudable state to displace metal laterally to collapse shrink pores and thereby densify the metal. The cores form hollows in the casting that reduce the extent of machining required to complete a bore through the casting, which bore is surrounded by metal densified by the core members.